The present invention relates to an optical coherent communication system, and, particularly to a technique for controlling intermediate frequency in an optical heterodyne communication system.
The optical heterodyne communication (optical coherent communication) system is advantageous in that a long distance, high density transmission is possible because reception sensitivity is much higher compared with a direct detection communication system and because frequency utilization efficiency is high.
In a heterodyne detection system, an optical detector receives a composite signal of an input optical signal transmitted from a transmitting portion and a local oscillation light from a local oscillation light source included in an optical signal receiving portion thereof. As a result, a beat corresponding to a frequency difference therebetween appears at an output of the optical detector as an electric intermediate frequency (IF) signal. By demodulating this intermediate frequency signal, a base band signal is obtained. When the relative frequency difference is not maintained constant, fluctuation of the intermediate frequency occurs, resulting in an error in a demodulated signal. Therefore, in the optical heterodyne detection, the intermediate frequency has to be stabilized.
Stabilization of intermediate frequency is disclosed in S. D. Lowney et al., "Frequency Acquisition and Tracking for Optical Heterodyne Communication Systems", Journal of Lightwave Technology, April 1987, pages 538 to 550. In this article, an IF band is divided into a plurality of subbands and a corresponding number of bandpass filters are provided for the respective subbands. An oscillation frequency of a local optical oscillation source is controlled while monitoring outputs of the respective bandpass filters to stabilize intermediate frequency. Since, however, a frequency variation of a light source is much wider than a predetermined intermediate frequency band and thus a large number of such bandpass filters are required, the Lowney et al. system is disadvantageous economically.
In the heterodyne detection system, frequency of local oscillation light is set to a value which is higher or lower than the optical input signal frequency. Assume that the local optical oscillation frequency f.sub.LO is set to a value higher than an input optical signal frequency f.sub.in for normal operation, i.e., f.sub.LO &gt;f.sub.in. When the optical input signal frequency becomes higher than the local optical oscillation frequency for a reason that the input optical signal frequency is varied remarkably or an oscillation frequency on a transmission side is deviated remarkably from a predetermined value the, intermediate frequency becomes the so-called image band IF. Although a value itself of the image band intermediate frequency is the same as that of a real band intermediate frequency, input frequency vs. output voltage characteristics of a frequency discriminator circuit used for automatic frequency control (AFC) must be reversed for the image and real band IF's. Therefore, when the intermediate frequency is within the image band, the AFC can not operate normally and becomes unstable.
This phenonmenon will be described in more detail with reference to FIGS. 1A and 1B.
FIG. 1A shows a case where the intermediate frequency spectrum is within a real band (f.sub.LO &gt;f.sub.in), in which a curve 40 is an input intermediate frequency (f.sub.LO -f.sub.in) vs. output voltage characteristic of a frequency discriminator circuit. In this case, when an input IF spectrum 41 is shifted rightwardly from the shown position, a d.c. component of the frequency discriminator circuit output becomes positive. A stabilization of the intermediate frequency is obtained by controlling a frequency of the local optical oscillation source such that it is lowered when the d.c. component of the frequency discriminator output becomes positive and increased when the output becomes negative.
On the contrary, when the intermediate frequency spectrum is within the image band (f.sub.LO &lt;f.sub.in), such control produces a problem to be described. FIG. 1B shows a case where the intermediate frequency spectrum is within the image band in which a curve 40' shows an input intermediate frequency (f.sub.LO -f.sub.in) vs. Output voltage characteristic of the frequency discriminator circuit. In this case, when the input IF spectrum 41' is shifted leftwardly from the shown position, a d.c. component of an output of the frequency discriminator circuit becomes positive and so the oscillation frequency of the optical local oscillator is lowered. However, with such reduction of the optical local oscillation frequency, the IF spectrum is further shifted leftwardly, eventually causing the IF spectrum to become out of the operation range of the frequency discriminator circuit. That is, the AFC is stabilized so long as the intermediate frequency is within the real band while it becomes highly unstable when it is within the image band.
The Lowney et al. method mentioned previously is based on an assumption that intermediate frequency is within a real band. Therefore, it is impossible to solve the problem occurring when the intermediate frequency is within the image band.